For 70 years the sounds of Princeton's a cappella groups have been echoing in arches across campus. At this year's Reunions scheduled appearances are planned at 1879 Arch for Thursday, May 26, (Koleinu at 6:30 p.m.) and Friday, May 27, (Wildcats at 9:30 p.m.; Tigerlillies at 10 p.m.; Roaring 20 at 11 p.m.; Nassoons at midnight).